remove sprint from ethermedium

Reference: /n/atom/patch/applied/mediumsprint
Date: Sun Aug 16 18:54:13 CES 2015
Signed-off-by: quanstro@quanstro.net

--- /sys/src/nix/ip/ethermedium.c	Sun Aug 16 18:54:03 2015
+++ /sys/src/nix/ip/ethermedium.c	Sun Aug 16 18:54:05 2015
@@ -414,7 +414,7 @@
 	int version;
 
 	version = multicastea(mac, a);
-	sprint(buf, "addmulti %E", mac);
+	snprint(buf, sizeof buf, "addmulti %E", mac);
 	switch(version){
 	case V4:
 		er->cchan4->dev->write(er->cchan4, buf, strlen(buf), 0);
@@ -436,7 +436,7 @@
 	int version;
 
 	version = multicastea(mac, a);
-	sprint(buf, "remmulti %E", mac);
+	snprint(buf, sizeof buf, "remmulti %E", mac);
 	switch(version){
 	case V4:
 		er->cchan4->dev->write(er->cchan4, buf, strlen(buf), 0);